1. Field of the Invention
Embodiments of the present invention generally relate to methods of thermally processing a material such as a semiconductor substrate.
2. Description of the Related Art
A number of applications involve thermal processing of semiconductor and other materials, which require precise measurement and control of the temperature of the material. For instance, processing of a semiconductor substrate requires precise measurement and control of the temperature over a wide range of temperatures. One example of such processing is rapid thermal processing (RTP), which is used for a number of fabrication processes, including rapid thermal annealing (RTA), rapid thermal cleaning (RTC), rapid thermal chemical vapor deposition (RTCVD), rapid thermal oxidation (RTO), and rapid thermal nitridation (RTN). In the particular application of CMOS gate dielectric formation by RTO or RTN, thickness, growth temperature, and uniformity of the gate dielectrics are parameters that influence the overall device performance and fabrication yield. Some of these processes require variations in temperature across the substrate of less than a few degrees Celsius.
It is desirable to obtain temperature uniformity in a substrate during thermal processing of the substrate. Temperature uniformity provides uniform process variables on the substrate (e.g. layer thickness, resistivity, etch depth) for temperature activated steps such as film deposition, oxide growth and etching. In addition, temperature uniformity in the substrate is necessary to prevent thermal stress-induced substrate damage such as warpage, defect generation and slip. For example, at 1150° C., a center to edge temperature difference on a four-inch silicon wafer of approximately 5° C. can induce dislocation formation and slip.
Temperature gradients may also be induced by other sources. For example, a substrate may have non-uniform emissivity because of spatial modifications to surface areas or volumes of the substrate. These modifications may include films that have been patterned by photolithography or locally doped regions, such as buried layers for bipolar transistors. In addition, substrate temperature gradients may be induced by localized gas cooling or heating effects related to processing chamber design as well as non-uniform endothermic or exothermic reactions that may occur on the substrate surface during processing.
In addition to minimizing temperature non-uniformity between all regions of a substrate during rapid thermal processing, it is also important that the actual time-temperature trajectory of a substrate varies as little as possible from the desired time-temperature trajectory of the substrate, particularly from the peak temperature. Time-temperature trajectory and peak temperature are described below in conjunction with FIG. 1.
FIG. 1 illustrates an ideal time-temperature trajectory, hereinafter referred to as target time-temperature trajectory 100 for an exemplary rapid thermal process, in this case a spike anneal process. The abscissa represents time, the ordinate represents substrate temperature, and target time-temperature trajectory 100 represents the desired temperature of a substrate at any time during the spike anneal process. At time 120, thermal processing of the substrate begins with the substrate at temperature 130, which is substantially room temperature. The substrate temperature is increased to temperature 132 during initial temperature ramp 201, described below in conjunction with FIG. 2. The substrate temperature is increased using high-intensity lamps, described below in conjunction with FIG. 3 as part of an exemplary RTP chamber. Starting at time 122, the substrate temperature is held constant at temperature 132 for the duration of stabilization period 202. At time 123 the substrate temperature is quickly increased to peak temperature 133 at time 124 and then immediately reduced to temperature 134 at time 125. If peak temperature 133 of a substrate is not met during RTP, important processes on the substrate may not be completed, such as post-implant anneal. If peak temperature 133 is exceeded during RTP, the process may be detrimentally affected in other ways, for example from unwanted diffusion of implanted atoms into the substrate or by exceeding the thermal budget of devices formed on the substrate.
FIG. 2 is a flow chart illustrating a typical process sequence 200 for a rapid thermal process, such as the spike anneal process described above in conjunction with FIG. 1. Generally such a process begins with an initial temperature ramp 201 of the substrate. Until the substrate is at a temperature of about 300° C. to about 400° C., open-loop heating is performed during the first segment of initial temperature ramp 201. Referring back to FIG. 1, open-loop heating takes place between time 120 and 121. During open-loop heating, no substrate temperature feedback is incorporated into controlling the process and instead lamp power is applied to the substrate at pre-determined values for a pre-determined duration in order to heat the substrate to a temperature regime in which the substrate will be substantially opaque to the majority of lamp energy being applied to it. Below about 300° C. a typical RTP substrate, such as a silicon wafer, is largely transparent to much of the radiant energy produced by typical heating lamps. When this is the case, radiant energy that passes through the substrate may then be detected by the pyrometers that measure substrate backside temperature, producing an inaccurate substrate temperature measurement. For a closed-loop heating control algorithm, inaccurate substrate temperature measurement may result in serious control problems during the initial temperature ramp 201, such as instability and/or hunting. Open-loop heating is typically used at the beginning of RTP to avoid this problem. The set points for open-loop heating are generally determined empirically.
After the substrate is heated to between about 300° C. to about 400° C., the initial temperature ramp 201 is then generally completed using a closed-loop control algorithm to bring the substrate temperature to a stabilization temperature of about 500° C. to about 700° C. Closed-loop control incorporates temperature measurement of the substrate at a given time step in the thermal process in one or more pyrometer zones into the control algorithm in order to fine-tune the power-output of the heating lamps for the subsequent time step. Time steps may be relatively small, for example 0.1 or 0.01 seconds. The minimum time step size is generally limited by the sampling rate of the temperature sensors used to control the heating process. The use of closed-loop control minimizes error between desired and actual substrate temperatures.
Because precise temperature control is so important in RTP, model-based control algorithms are sometimes used instead of conventional PID control algorithms as part of closed-loop temperature control during thermal processing. A model-based control algorithm—also known as a model-based controller—may further reduce the error between actual substrate temperature and the target temperature during processing compared to a standard PID control loop. Rather than responding to a measured error by changing an input (in this case lamp power) that is proportional to the measured error, a model-based control algorithm applies an energy transfer model of the substrate and chamber at each time step to predict how the substrate will behave thermally in the next time step. Not unlike a simulation program, the model-based controller includes all relevant aspects of the system into the heat transfer model, including the optical properties of the substrate and chamber walls, the power output and location of the lamps, etc. The controller considers all these factors and then controls the power output of each lamp or group of lamps based on the current temperature of the substrate, the desired temperature and the modeled response of the entire system. This predictive method allows for less variation from a target temperature during thermal processing-especially when the target temperature is changing quickly as a function of time-making it useful for spike anneal and related rapid thermal processes.
A notable drawback of model-based controllers is that their ability to precisely control a substrate temperature to a desired time-temperature trajectory is limited by the accuracy of the assumptions on which the model is based. The most important factors that need to be accurately taken into account by the model-based controller are aspects of the process chamber, such as the size and shape of the chamber, the lamp power, etc., and the optical properties of the substrate, including substrate emissivity, absorptivity, reflectivity and transmissivity. “Emissivity,” “absorptivity,” “reflectivity” and “transmissivity,” as used herein, refer to those properties of a material for light wavelengths between about 0.2 μm and about 5 μm. A model-based controller may be corrected through testing to more accurately represent a particular RTP chamber's behavior during thermal processing; empirical factors, also known as “fudge factors”, are typically included in model-based control algorithms and may be determined through trial and error to fine-tune a controller's accuracy. However, because a controller's calculations are based on a specific substrate type with fixed optical properties, the model-based algorithm will be inherently inaccurate for all but a single type of substrate. This is because the optical properties of different substrates vary dramatically depending on how the substrate has been processed prior to RTP. For example, highly reflective substrates, such as metallic substrates, and light-absorbing substrates, such as heavily patterned substrates, may both be processed in the same RTP chamber using the same control algorithm. Unless a different controller has been developed to account for each substrate type being processed, sub-optimal temperature control will result. Multiple controllers may be programmed for use in a single RTP chamber, however each controller requires fine-tuning based on repeated experiments. In some situations, for example in a semiconductor foundry, there may be hundreds of different substrate types processed in a given RTP chamber and each type may have its own unique combination of optical properties. This makes optimizing a specialized controller for each substrate type to be processed impracticable.
Referring back to FIG. 2, once the substrate reaches stabilization temperature at the end of initial temperature ramp 201, a stabilization period 202 generally takes place after. The stabilization period 202 is intended to eliminate thermal gradients that have been imprinted on the substrate during the initial temperature ramp 201 by allowing the substrate to equilibrate prior to beginning spike anneal 203, which is the temperature sensitive segment of the thermal process. Non-uniformities in substrate temperature that are present at the beginning of spike anneal 203 are unlikely to be corrected during the process. The stabilization period 202 is between about 5 seconds and about 30 seconds in length, typically between about 10 seconds and about 20 seconds. The substrate temperature is controlled to remain at the stabilization temperature 132, as illustrated in FIG. 1, which may be between about 500° C. to about 700° C., depending on the particular thermal process.
Upon completion of the stabilization period 202, spike anneal 203 then begins. In this example, spike anneal 203 is the segment of the process in which the thermal processing of the substrate actually takes place. A preferred application of the spike anneal process is to perform the anneal on a substrate after boron implant. In this case, spike anneal 203 relocates the implanted boron from random locations in the crystal to electrically active sites in the silicon lattice while minimizing the thermal exposure of the substrate. As illustrated in FIG. 1, spike anneal 203 takes place between times 123 and 124 and is followed by a cool-down 204. Ramp rates of the substrate temperature during spike anneal 203 are generally between about 150° C./s and about 300° C./s and peak temperature 133 is about 1000° C. to about 1200° C./s, meaning that spike anneal 203 generally only last a few seconds. Hence, there is little time for a control algorithm to correct variation in substrate temperature from the target time-temperature trajectory 100 during the most temperature sensitive segment of the thermal process. Any variation that occurs in substrate temperature from the target temperature during this segment of the rapid thermal process, e.g. overshoot, undershoot or widespike, will reduce the peak temperature repeatability between substrates. Referring back to FIG. 2, spike anneal 203 is then followed by cool-down 204, ending the rapid thermal processing of the substrate.
As noted above, it is important for temperatures to be as uniform as practicable in a substrate during thermal processing of the substrate. In practice, the edge region of a substrate is thermally affected by the periphery of the RTP chamber more than are other regions of the substrate, leading to chronic temperature non-uniformities residing in the edge region. Standard control algorithms are designed to respond to radial temperature non-uniformities once detected. For very short processes, such as spike anneal processes, the control algorithm may not be able to compensate quickly enough, resulting in temperature non-uniformities near the edge of the substrate. Further, because the current design of RTP chambers is geared toward radial temperature non-uniformity on a circular substrate, this method of temperature control is unable to correct non-radial temperature non-uniformities, for example a “cold spot” that is not symmetrically centered on the substrate.
Thus, there is still a need for methods and apparatus for controlling the rapid thermal processing of a substrate that may be used for a wide range of substrates, that requires minimal tuning, that improves peak temperature repeatability, that allows closed-loop control to be used at lower temperatures and that minimizes temperature non-uniformity on the substrate.